The present invention relates to fire alarm systems, and more particularly, to a fire alarm system in which the raising of false fire alarms is prevented, but with which a fire alarm can be issued in the early stage of a true fire.
Fire sensors in a conventional fire alarm system are sometimes erroneously operated by electrical noise, tobacco smoke rising momentarily, or a blast of wind. In order to prevent the difficulty of a false fire alarm being issued because of the erroneous operation of the fire sensor, a fire alarm system has been proposed in which, on the side of the fire signal receiver, after a fire sensor is operated, the fire sensor is reset so that a fire alarm is issued only after the fire sensor has operated plural times within a predetermined period of time (see Japanese Published Patent Application No. 36119/1976).
In addition, a fire alarm system has been proposed in which a plurality of fire sensors are installed in a monitoring area, and a fire alarm is issued only when at least two fire sensors are operated (see Japanese Laid-Open Patent Application No. 146594/1977).
As is apparent from the above description, in the conventional fire alarm system in which the issuance of false fire alarms is prevented, there is always a time delay before the alarm can be issued in an actual emergency. That is, the fire alarm system in which a fire alarm is issued in response to signals outputted by at lest two fire sensors is disadvantageous in that the fire alarm can be issued only after the fire has been spread because, in the initial stage of the fire, only one fire sensor is operated. Furthermore, the dual system in which two fire sensors are provided in a monitoring area is intricate in installation, and accordingly high in installation cost.